The present invention relates to a semiconductor laser used as a light source for an optical pickup for a video disc, a digital video audio disc (DAD), a writable, readable and erasable optical disc, and the like. As shown in FIG. 1, a three-beam pickup has been heretofore proposed for use in optical systems of the types described. In the drawing, reference numerals l, 3 and 4 designate, respectively, a semiconductor laser, a diffraction grid for diffracting a laser beam 2, and a collimating lens for converting the diffracted laser beam into a parallel light flux. Reference numerals 5, 6 and 7 denote a polarized prism type beam splitter, a quarter-wave plate for rotating the polarization axis of the laser beam through an angle of 90 degrees, and an objective lens for focussing the laser beam on a surface of the optical disc 8 coated with a film 8'. In recording is to be effected, a modulator 1' should be provided to modulate the output from the laser 1.
A cylindrical lens focusses a return beam which is separated from the laser beam 2 by the polarized prism-type beam splitter 5, onto laser beam receptive elements 11 and 12.
As shown in FIG. 1, the laser beam 2 emitted from the semiconductor laser 1 is diffracted by the diffraction grid 3 and is then divided into three laser beams. These three beams, which include a zero-order diffraction beam 13 and positive and negative diffraction beams 14, are converted by the collimating lens 4 to parallel luminous fluxes, which then pass through the beam splitter 5. The polarization axes of the three laser beams 13 and 14 are rotated through an angle of 90 degrees by the quarter-wave plate 6, and then focussed to a specified beam diameter by the objective lens 7 on the pits 9 on the surface of the optical disc, as shown in FIG. 3. The laser beams 13 and 14 are selectively reflected from the pitted surface and reoriented through an angle of 90 degrees by the beam splitter 5 and focussed on the beam receptive elements by the cylindrical lens 10.
FIG. 2 is an end view showing the three laser beam spots focussed onto the pits 9 of the optical disc 8. The central, zero-order diffraction beam 13, the reflected portion of which is focussed on the beam receptive element 11, is used to transfer the main signal, and is also used for a servo focussing function. The positive and negative primary diffraction beams 14, the reflected portions of which are focussed on the beam receptive element 12, are directed before and behind the zero-order diffraction beam 13. These beams 14 are employed to effect a servo tracking operation whereby the zero-order diffraction beam 13 is prevented from being deflected from a track 15 of the pits 9.
In the conventional optical pickup system described above, a diffraction grid is used to divide a laser beam into three beams, whereas the collimating lens is employed for converting the three beams into parallel luminous fluxes. To this end, a relatively large space is required for the diffraction grid and the collimating lens, thus making the overall optical system long and the size of the pickup large.
Further, another disadvantage in the prior art approach is that the required diffraction grid and the collimating lens are expensive components.
For recording onto a disc, with further reference to FIG. 3, the pits 9 formed on the surface of the optical disc typically have a width of 0.5 .mu.m, a length of 0.9 to 3.3 .mu.m, and a track pitch of 1.1 .mu.m. The spot diameter d of the laser focussed in the aforementioned manner on a pit 9 may be expressed by d.varies..lambda./N.A., where the wavelength of the beam and the aperture number of the lens are .lambda. and N.A., respectively.
From the aforementioned equation it can be seen that, in order to reduce the spot diameter d of the laser beam, it is necessary to reduce the wavelength .lambda. or increase the aperture number N.A.
If it is desired to increase the pit density, the spot diameter d must of course be reduced. This is made difficult, however, due to the critical short wavelength of GaAs semiconductor lasers, about 7400 .ANG.. Hence, an entirely new type of semiconductor laser would be required. Moreover, increasing the aperture number above 0.5 gives rise to a problem of aberration and renders the design of the associated lenses difficult.